Discovery of a 24-millisecond pulsar in a very long orbit with the Murchison Widefield Array

Abstract: We report the discovery of PSR J0125$-$5854, a pulsar with a spin period of 24 ms and a dispersion measure of 11.66 pc cm$^-3$ in the ongoing Southern-sky MWA Rapid Two-metre (SMART) survey with the Murchison Widefield Array (MWA). The pulsar is located at a high Galactic latitude of $-57^{\circ}$, and at a distance of 0.5$\text{-}$1 kpc per the Galactic electron density models. Follow-up observations with the MWA and MeerKAT telescopes have revealed that this pulsar is in a binary system with an orbital period of more than 290 days, and a steep spectrum (flux density, $ S \propto ν^α $, where $ν$ is frequency and $ α= -2.2 \pm 0.3 $). Analysis of current observational data hints at a potential binary configuration with an orbital period of $833.60 \pm 0.04$ days, a projected semi-major axis of $241.36 \pm 0.05$ light-seconds, and a minimum companion mass $0.4152 \pm 0.0001$ M$_\odot$, with a low eccentricity orbit of $0.0052 \pm 0.0006$. We discuss the potential formation channels for this system, and conjecture that the companion is likely a Helium white dwarf. Further observations are required in order to better constrain the orbital and spin parameters. We discuss the implications of this discovery, which emerged after processing a small fraction of survey data, on the prospects of finding more millisecond pulsars with the SMART survey, and with future surveys planned with the low-frequency SKA-Low.

• The paper presents the discovery and characterization of PSR J0125-5854, a 24ms pulsar in an 833-day orbit with a helium white dwarf companion.
• It employs deep, low-frequency surveys and multi-telescope strategies, using MCMC and TEMPO modeling to precisely determine orbital parameters.
• The spectral analysis reveals a steep index (α = -2.2) with significant scintillation, underscoring survey optimizations for SKA-Low era pulsar searches.

Discovery and Characterization of PSR J0125-5854: A 24 ms Pulsar in a Wide Orbit

Introduction

The paper details the discovery and comprehensive analysis of PSR J0125-5854, a millisecond pulsar (MSP) with a spin period of 24 ms, located at a high Galactic latitude ($b = -57^\circ$) and an estimated distance of 0.5–1 kpc, detected via the Southern-sky MWA Rapid Two-metre (SMART) survey using the Murchison Widefield Array. The pulsar exhibits characteristics placing it within the class of mildly recycled systems found in wide binaries, and follow-up timing establishes its orbital period at $833.60 \pm 0.04$ days, with a low eccentricity ($e = 0.0052 \pm 0.0006$) and a minimum companion mass of $0.4152 \pm 0.0001 \, M_\odot$. The available evidence strongly supports a helium white dwarf (He WD) as the companion.

Survey Strategy and Observational Methodology

The SMART survey leverages the large field of view and voltage capture capabilities of the MWA. Observations occur at central frequencies of 154–184 MHz with beamforming enabling positional localization to sub-arcminute precision. Discovery integration times spanned 80 minutes, significantly longer than previous high-latitude surveys, boosting sensitivity to low-DM, high-latitude MSPs. Initial detection was confirmed via independent and archival observations, exploiting the survey's spatial overlap, and followed up with the MWA, MeerKAT, and Parkes telescopes.

MWA observations involved beamforming strategies using hexagonal grids, while the MeerKAT follow-up comprised multiband high-resolution observations (UHF, L, S1), utilizing densely tiled tied-array beams and SeeKAT localization algorithms. This led to a final positional accuracy of $\sim 2"$, with MeerKAT detections showing significant S/N variability due to strong scintillation.

Timing and Orbital Parameter Determination

Spin period analysis across multiple epochs revealed significant variations inconsistent with isolated spin-down rates, indicative of orbital Doppler modulation. The orbital solution was derived via Markov Chain Monte Carlo (MCMC) fits to timing data and refined with TOA modeling using the TEMPO framework. The resulting orbital parameters are:

• Orbital period: $P_b = 833.60 \pm 0.04$ days
• Projected semi-major axis: $x = 241.36 \pm 0.05$ light seconds
• Minimum companion mass: $M_c \geq 0.4152 \pm 0.0001\, M_\odot$
• Eccentricity: $e = 0.0052 \pm 0.0006$
• Longitude of periastron: $\omega = 152^\circ \pm 4^\circ$

The system is typified by minimal eccentricity and a companion mass consistent with a canonical He WD, validated against theoretical period-mass and period-eccentricity relationships for binaries formed via Roche-lobe overflow during giant branch evolution (2606.18823).

Spectral Properties and Scintillation

Flux densities were measured across four frequency bands ($833.60 \pm 0.04$0150 MHz to $833.60 \pm 0.04$13 GHz) using MWA and MeerKAT data, calibrated via SEFD modeling and radiometer equations. The broadband spectrum is steep, with a spectral index $833.60 \pm 0.04$2, placing PSR J0125-5854 among the steepest-spectrum MSPs known. Strong scintillation was observed, with a measured scintillation bandwidth $833.60 \pm 0.04$360 MHz at 0.54 kpc; the implied turbulence strength ($833.60 \pm 0.04$4) is comparable to those seen in nearby MSPs such as PSR J0437-4715.

Binary System Analysis and Comparative Population Study

With only $833.60 \pm 0.04$520 binary pulsars in the Galaxy exhibiting orbital periods $833.60 \pm 0.04$6 days, PSR J0125-5854 adds to the rare population of wide-orbit, mildly recycled MSPs with He WD companions. The mass–period relation follows expectations from binary evolutionary models for Population I–II progenitors, while the measured eccentricity slightly exceeds the predicted value for similar systems, indicating possible variations in initial binary conditions or subsequent perturbations.

Comparison with analogous systems (e.g., PSRs J0214+5222, J0407+160) confirms the formation channel via short-duration mass transfer at the giant branch tip, resulting in partial rather than full recycling and hence longer spin periods ($833.60 \pm 0.04$7 ms). These distinctions delineate this system from high-eccentricity, main-sequence star binaries (e.g., PSR B1259-63, J2032+4127) and canonical non-recycled pulsars.

Implications for Pulsar Surveys and Galactic MSP Demographics

The discovery process and subsequent characterization of PSR J0125-5854 demonstrate the efficacy of wide-field, deep, low-frequency surveys (SMART) in recovering rare, steep-spectrum MSPs at high Galactic latitudes previously missed due to short dwell times or insufficient sensitivity of earlier surveys (e.g., Parkes 70-cm and HTRU). This underscores the importance of survey strategy optimization—long dwell times and full-sky coverage—for SKA-Low era pulsar searches.

Population simulations predict up to 55 MSPs within DM $833.60 \pm 0.04$8100 pc cm$833.60 \pm 0.04$9 from the completed SMART survey, with $e = 0.0052 \pm 0.0006$015 new discoveries. The specificity of the detection pipeline (no acceleration searches) bias towards isolated and wide-orbit binary MSPs. The complementarity with LOFAR LOTAAS and MeerKAT MMGPS surveys is significant, offering full multi-frequency coverage and rapid follow-up capabilities.

Optical Counterpart Identification

No counterpart was found in GAIA at the expected apparent magnitude for a main-sequence companion. However, a DESI Legacy survey source (r = 22.03, $e = 0.0052 \pm 0.0006$1), consistent with a He WD at $e = 0.0052 \pm 0.0006$2–$e = 0.0052 \pm 0.0006$3 kpc, is located within the timing uncertainty region, bolstering the He WD companion hypothesis.

Conclusion

PSR J0125-5854 is a newly discovered MSP with a spin period of 24 ms in an exceptionally long-period, low-eccentricity binary orbit. The companion mass and spectral characteristics support its classification as a mildly recycled pulsar with a helium white dwarf companion. The methodologies detailed—in survey strategy, localization, orbital modeling, and spectral profiling—showcase the current state of time-domain radio astronomy and its capabilities in dissecting binary pulsar populations. The results enhance constraints on binary formation channels, period–mass, and period–eccentricity relations, and inform optimized survey designs for SKA-Low and beyond. Further timing and multi-wavelength observations are required to achieve phase-coherent solutions, better constrain orbital and companion properties, and probe the evolutionary deviations hinted by the eccentricity analysis.

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Paper referenced: "Discovery of a 24-millisecond pulsar in a very long orbit with the Murchison Widefield Array" (2606.18823).